Impedance matched stripline ferrite y circulator having increased ground plane spacing at the junction of the center conductors



Nov. 28, 1967 K. L. CARR 3355679 IMPEDANCE MATCHED STRIPLINE FERRITE Y CIRCULATOR HAVING INCREASED GROUND PLANE SPACING AT THE NDUCTORS JUNCTION OI THE CENTER CO 3 Sheets-Sheet 1 Filed March 20, 1964 PROR ART INVENTOR. KENNETH L. CARR BY WZfiyM/W @Mz! fairwa ATTORNEYS NOV. 28, K C IMPEDANCE MATCHED STRIPLINE FERRITE Y CIRCULATOR HAVING INCREASED GROUND PLANE SPACING AT THE JUNCTION OF THE CENTER CONDUGTDRS Filed March 20, 1964 3 Sheets-Sheet 2 )\PR|OR ART p INVENTORL LmM hA 36 v KENNETH 1.. CARR /m m/z @Mij maf/frdf ATTO R N EYS Nov. 28, 1967 K. L. CARR 3,355,679

. IMPEDANCE MATCHED STRIPLINE FERRITE Y CIRCULATOR HAVING INCREASED GROUND PLANE SPACING AT THE JUNCTION OF THE CENTER CONDUCTORS Filed March 20, 1964 mununf INVENTCR. KENNETH L. CARR 5 Sheets-Sheet 5 flrga (fii Mai EWW ATTORNEYS wave ferrite device in a manner considered in conjunction taken along the plane 3-3 United States Patent Ofifice 3,355,679 lh'iPEDANE MATCHED STRIPLENE FERRITE Y CIRCULATGR HAVING INCREAED GROUND PLANE SPACING AT THE JUNCTION OF THE CENTER QUNDUCTORS Kenneth L. Carr. Bedford, Mass, assignor to Ferrotec, Inc, Newton, Mass, a corporation of Massachusetts Filed Mar. 20, 1954, Ser. No. 353,357 1 Claim. (Cl. 333-11) This invention relates generally to devices used in the transmission of electromagnetic wave energy whose frequency is in the microwave region of the electromagnetic wave spectrum and more particularly concerns devices employing ferrites to control the transmission of microwave energy.

The invention is an improvement upon ferrite devices of the type utilizing coaxial or strip transmission line and more generally relates to ferrite devices, such as the coaxial isolator and the stripline circulator, in which the wave energy field is between a center conductor and an outer conductor or ground plane.

In the transfer of wave energy from one point to another within the conventional microwave ferrite device, the transfer path has impedance discontinuities in it that have deleterious facts upon the wave energy transmission. In order to make the transfer path electrically smooth, it is customary to employ impedance matching transformers having one or more quarter Wave steps. The impedance matching transformer increases the size of the microwave device and because of the quarter wave steps the transformer is inherently frequency sensitive and, therefore, impairs the fitness ofthe device for use over a broad band of frequencies. The impedance discontinuity in the Wave energy transfer path occurs in the conventional microwave ferrite device atrthose places where the medium in which the wave energy propagates undergoes a transition from a material of one dielectric constant to a material having a different dielectric constant. For example, an impedance discontinuity occurs in the conventional microwave ferrite device at that transition where wave propagation changes from an air filled coax al or strip transmission line to a dielectrically loaded coaxial or strip transmission line and at that transition where the change is reversed.

The invention resides in microwave ferrite devices constructed so that the transitions in the transmission path .are inherently matched. The invention eliminates the impedance matching transformer of the conventional microresulting in a smaller device. Because the transmission path is, in the invention,

. inherently matched, the smaller device has better broad band characteristics than the larger conventional device.

The arrangement and mode of operation of the invention in its different embodiments can be more fully understood from the exposition which follows when it is with the accompanying drawings in which:

FIG. 1 depicts a coaxial line ventional construction;

FIG. 2 illustrates a coaxial resonance isolator in which resonance isolator of conj internal matching is achieved by reducing the diameter of the center conductor;

FIG. 3 is a cross sectional view of the device of FIG. 2

FIG. 4 depicts a coaxial resonance isolator in which internal matching is achieved by enlarging the diameter of the outer conductor;

FIG. 5 shows the construction of a strip line resonance isolator utilizing the conventional quarter wave stepped dielectric transformer;

FIG. 6 is an end view of the conventional device illustrated in FIG. 5;

FIG. 7 depicts a strip line resonance isolator in which internal matching is attained by narrowing the width of the center conductor;

FIG. 8 is a cross-sectional view of the FIG. 7 device;

FIG. 9 illustrates a strip line resonance isolator in which internal matching is attained by increased ground plane spacing;

FIG. 10 is a partially exploded View of a strip line Y circulator utilizing conventional dielectric matching rings;

FIG. 11 depicts a strip line Y circulator, constructed in accordance with the invention, in which internal matching is achieved by decreased width of the center conductors at the junction; and

FIG. 12 depicts a strip line Y circulator, constructed in accordance with the invention, in which internal matching is achieved by an increase in ground plane spacing.

A conventional ferrite resonance isolator is illustrated in FIG. 1, the isolator being of the type utilizing coaxial transmission line. The characteristic impedance of air filled coaxial transmission line is standardized at some fixed value and it is assumed, for purposes of exposition, that the characteristic impedance of the depicted air filled coaxial line is 50 ohms. Situated within the coaxial line, between the center conductor 1 and the outer conductor 2 are ferrite bars 3 and d, which extend longitudinally along the line for a length Z. The coaxial line, for at least the length l is half filled with a dielectric material 5, such as aluminum oxide, so that the dielectric material fills a sector of the cross-section of the coaxial line. The function of the aluminum oxide dielectric is to distort the TEM mode of the wave energy propagating along the line. The dominant TEM mode of air filled or exhausted coaxial transmission line has no longitudinal mag netic field component and does not, therefore, provide the rotating magnetic field required to drive the ferrites spin system. In the absence of a longitudinal component of the magnetic field, the ferrite in the coaxial line cannot have non-reciprocal effects upon wave transmission. By loading the coaxial line anti-symmetrically with aluminum oxide, or a similar dielectric, the uniform transverse electric field of the TEM mode is distorted to obtain a longitudinal magnetic field component from the propagating wave energy. The electric field of the wave energy is intense within the dielectric while in the empty sector the electric field intensity is very low. That is, the electric field is caused to be concentrated within the dielectric with a resulting reduction of intensity in the empty sector of the coaxial line. At the interface of the aluminum oxide dielectric and the air in the line, there is a large gradient in the electric field intensity and consequently a large longitudinal magnetic field component exists at the inter face.

In half-filling the coaxial line with aluminum oxide, the characteristic impedance for that segment of the line is changed from 50 ohms to somewhat less than half that value, viz., 23 ohms. An abrupt change in the lines characteristic impedance causes reflections of the wave energy propagating along the lines and to smooth out the impedance change, the ends of the aluminum oxide dielectric are stepped to form a quarter wave transformer. The quarater wave length of the step refers to the length r of a wave at some specified frequency; at any other frequency the step is something other than a quarter wave in length. The quarter wave transformer is, for that reason, frequency sensitive and a degradation in performance is encountered as the frequency departs from the specified frequency at which the steps are a quarter wave long.

The embodiment of the invention shown in FIG. 2 is an improvement upon the conventional coaxial resonance 3 isolator. The FIG. 2 embodiment provides a device that is inherently internally matched and as the frequency sensitive matching transformer is eliminated the improved device can operate effectively over a broader frequency hand than can the conventional coaxial isolator.

The resonance isolator of FIG. 2 employs a coaxial line having an inner conductor 16 and an outer conductor 11. A segment 12 of the coaxial line of length l is half filled with a dielectric material 13, such as aluminum oxide. As in the conventional coaxial isolator, the aluminum oxide dielectric fills a 183 sector and distorts the TEM mode to provide a longitudinal magnetic field component. A pair of elongated ferrite bars are situated on opposite sides of the center conductor, upon the aluminum oxide dielectric. For the length of segment 12, the diameter of the inner conductor 13 is reduced to cause the characteristic impedance of the segment 12 to match the characteristic impedance of the air filled coaxial line extending on both sides of the segment. That is, assuming the characteristic impedance of the air filled coaxial line is 50 ohms, the aluminum oxide dielectric in segment 12 causes the characteristic impedance to be reduced by half in that segment. By decreasing the diameter of the inner conductor 10 for the length of the segment, the characteristic impedance of the segment is restored to a value of 50 ohms. FIG. 3 is a section, taken along the plane 3--3 in FIG. 2, showing the reduction in diameter of the coaxial lines center conductor.

As an alternative to reducing the diameter of the inner conductor, the character impedance of the segment can be restored to the desired value by increasing the internal diameter of the outer conductor for the length of the segment. This alternative construction is shown in FIG. 4. The segment 14 of the coaxial line is half filled with the TEM mode distorting dielectric 15. Upon the faces of dielectric 15, on either side of the center conductor are secured dielectric ferrite bars 16 and 17. The inner conductor 18 of the coaxial line is of uniform diameter throughout the line. At the segment 14, the internal diameter of the outer conductor has been increased to cause the characteristic impedance of the segment 14 to match the characteristic impedance of the air filled line.

The characteristic impedance of a coaxial line is given approximately by the equation:

where:

Z is the characteristic impedance,

.2 is the dielectric constant of the medium in the line, D is the internal diameter of the outer conductor, and d is the diameter of the inner conductor.

By varying D or d, or both D and d, a change in e can be offset to maintain Z at a constant value. The dielectric constant e of the semi-filled segment of the coaxial line is fourfold or fivefold larger than the dielectric constant e of the air filled line. To maintain an impedance match between the semi-filled segment and the air filled line, the inner conductor of the segment may be reduced in diameter, or the internal diameter of the outer conductor of the segment may be increased, or a combination of a reduced inner conductor and an enlarged outer conductor may be employed.

The invention can, further, be embodied in a strip transmission line resonance isolator. The strip transmission line is analogous to the coaxial transmission line in that the strip line is a structure having an air dielectric and utilizes a center conductor in conjunction With energy confining means. In the coaxial line, the wave energy is confirmed by the tubular outer conductor while in the strip transmission line the wave energy is largely confined by ground plane plates.

FIGS. and 6 depicts a conventional strip line resonance isolator having ferrite slabs 21 and 22 disposed etween the center conductor 23 and ground plane plates 24 and 25. The ferrite slabs are secured to dielectric bars 26 and 27 which partially fill the space between the center conductor and the ground plane plates. Dielectric bars 26 and 27 distort the mode of wave propagation in the air filled strip transmission line to provide a longitudinal component of the wave energys magnetic field. Anti-symmetric loading of the strip transmission line by dielectric bars 26 and 27 provides the rotating magnetic field required to permit the ferrite in the line to have non-reciprocal effects upon wave transmission.

The characteristic impedance of the air filled strip line is altered by the presence of dielectric bars 26 and 27 for a segment of the line. The ends of the dielectric bars are stepped at intervals of 4 so as to act as a quarter wave impedance transformer. As in the conventional coaxial isolator, the quarter wave transformer of the strip line is frequency sensitive as a degradation in performance is encountered as the operational frequency departs from the specified frequency at which the steps are a quarter long.

The embodiment of the invention shown in FIGS. 7 and 8 is an improvement upon the conventional strip line resonance isolator. In the improved structure the quarter wave impedance transformers are eliminated and the characteristic impedance of structure is matched by decreasing the width of the center conductor 28 for the length of the segment of the line that is anti-symmetrically loaded by the dielectric 31 and 32 which carry the ferrite bars 33 and 34.

The characteristic impedance of a strip transmission line is given, approximately, by the equation:

where:

Z is the characteristic impedance e is the dielectric constant of the wave transmission medium b is the spacing between ground planes, and

w is the width of the center conductor.

The equation assumes that the thickness of the center conductor is negligible; that is, that the thickness of the center conductor is vanishingly small. From the equation it can be deduced that a change in characteristic impedance caused by an increase in the dieletcric constant e can be offset by reducing the value of w, the width of the center conductor while maintaining a constant spacing b between ground plane plates 29 and 30. The presence of dielectric bars 3-1 and 32 and ferrite bars 33 and 34 in the strip transmission line of FIGS. 7 and 8 increases the dielectric constant in that anti-symmetrically loaded segment of the line. Therefore, by decreasing the width of center conductor 28, the characteristic impedance of the dielectrically loaded line segment is matched to the air filled line.

Instead of decreasing the width of the center conductor, an impedance match can be obtained by increasing the spacing b between the ground plane plates. FIG. 9 is a horizontal view of a strip transmission line resonance isolator in which the spacing of the ground plane plates 35 and 36 has been increased for a segment of the line of length l. The line segment is anti-symmetrically loaded by dielectric bars, identical with bars 31 and 32 of FIG. 7, to which are attached ferrite slabs 37 and 38. The center conductor 39 is uniform throughout the strip line.

In order to maintain a constant characteristic impedance throughout the strip line resonance isolator, both the ground plane spacing b and the width w of the center conductor can be altered to offset the change in the dielectric constant e caused by the presence in the line of materials whose dielectric constant is different from that of air. Thus, an embodiment of the invention can be constructed that utilizes the decrease in Width (w) of the center conductor and the increased ground plane spacing z in the same device.

A conventional strip line Y circulator is depicted in FIG. 10 having a three branched center conductor 40 disposed between ground plane plates 41 and 42. Above and below the central portion of the center conductor 40 are ferrite discs, such as the disc 43. Each disc is fitted within a dielectric ring such as annulus 44, and the dielectric and ferrite assemblies till the spaces between the center conductor and the ground plane plates above and below the junction of the three branches. The dielectric rings match the impedance of the ferrite filled segment to the air filled strip line.

Each ground plane plate has a recess, such as the recess 45, to receive the pole piece of a magnet that establishes a magnetic field transversely through the ferrite discs. Each of the three branches of the center conductor terminates in a coaxial connector, such as connector 46.

FIG. 11 depicts an embodiment of the invention in which the dielectric matching rings are eliminated to obtain a device having improved performance. The width of the three branches of center conductor 48 is reduced at the symmertical junction sufiiciently to match the impedance of the ferrite filled segment to the impedance of the air filled line. As in the conventional Y circulator, ground plane plates 49 and 50 are provided with circular recesses, as at 51, to receive the pole pieces of a magnet that establishes a magnetic field which extends transversely tln ough the ferrite discs. By eliminating the dielectric matching rings, the embodiment of FIG. 11 can be of smaller size than a comparable conventional strip line Y circulator. From FIG. 11 it is evident that only the parts of the three branched center conductor that are sandwiched between the ferrite discs 52 and 53 are reduced in width; that is, the portions protruding away from the discs are of larger Width. Because the dielectric rings of the conventional device are eliminated, the coaxial connectors can be moved closer to the junction to make a smaller device.

FIG. 12 illustrates an embodiment of the invention in which matching is achieved by increasing the spacing between ground plane plates over the area occupied by the ferrite discs. In the embodiment of FIG. 12, the three branches of center conductor 54 are of uniform width. The junction of the center conductor is sandwiched between ferrite discs 55, 56 with each of the three branches extending away from the discs. Each of the ground plane plate 57 and 58 are counterbored, as at 59, to increase th spacing between the ground planes in the area of the junc tion. The counterbores are of such diameter as to receivl the discs and the discs are of sufiicient thickness to fill tilt volume between the center conductor and the grounc planes.

Modifications of the embodiments of the inventior depicted in the drawings may be made without departing from the essential concept of the invention and, indeed, are apparent to those skilled in the electronics art. For example, the embodiments of the invention shown in FIGS. 11 and 12 can be combined to produce a device that utilizes both a decrease in width of the center conductor and increased ground plane spacing to achieve the impedance match. It is intended, therefore, that the invention not be limited to the precise arrangements illustrated, but rather that the inventions scope be construed as delimited by the appended claim.

What is claimed is:

In a Y circnlator of the strip transmission line type having the junction of the center conductors disposed between a pair of ferrite members which, in turn, are disposed between a pair of ground plane plates, the improvement of matching the characteristic impedance of the ferrite loaded junction to the characteristic impedance of the strip lines contiguous to the junction, the improvement comprising an increase at the junction in the spacing of the ground plane plates.

References (Iited UNITED STATES PATENTS 3/1961 Locus 3339 1/1965 Drumheller et al. 333-9 X OTHER REFERENCES Ragan: Microwave Transmission Lines, McGraw-Hill, 1948, pp. 165, 183. I

Handbook of Tri-Plate Sanders Associates, 1956, page 

